How to watch green ‘comet A3’ whizzing through our skies

This autumn is turning into a feast for sky-watchers, with not one but two comets visible – and both potentially visible to the naked eye.

Both comets will glow green in our skies.

One comet, called C/2023 A3 (Tsuchinshan-ATLAS) is already visible, while another newly discovered comet could be visible towards the end of the month. Comet A3 will remain visible for much of October, with a spectacular tail that’s set to grow.

The second comet, known as C/2024 S1 (ATLAS) was spotted very recently, and is set to become visible later in the month, passing closest to Earth on 24 October and then returning (possibly) for Halloween.

Some experts predict the second comet could even outshine A3 – and be brighter than Venus in the night sky.

What is comet A3?

Comet A3 is already the brightest comet in years, and was last seen on Earth 80,000 years ago in the time of the Neanderthals.

It comes from the Oort Cloud, a region of icy debris at the very edge of the solar system, a common source of comets.

Like the second comet, it’s green due to the presence of diatomic carbon, which glows green in the light from the Sun.

It will be visible throughout this month.

Its long name – C/2023 A3 (Tsuchinshan-ATLAS) – is down to the fact it was first spotted independently by astronomers at two observatories: Asteroid Terrestrial-impact Last Alert System (ATLAS) spotted it on 22 February, 2023.

Astronomers at China’s Purple Mountain (Tsuchinshan) Observatory in China spotted the comet in images from January 9 2023, hence the name Tsuchinshan-ATLAS.

It completes an orbit around the Sun every 80,660 years, astronomers believe, and will be visible throughout this month.

How can I see comet A3 this month?

Over the next two weeks, it will be visible from the Northern Hemisphere, according to Star Walk magazine, being visible in Serpens Caput from October 15-19 and across the constellation Ophiuchus from 20-31 October.

The tail of the comet will grow in this period, possibly reaching up to 40 times the size of the Full Moon, Star Walk says.

Robin Scagell of Pop Astro says: “To start with it will only be visible shortly after sunset, but after about 14 October it will be visible for more than an hour, although still low down, so you’ll need a good horizon to the south-west to be able to see it.”

To find it, try using a star spotting app such as Star Walk – and it is worth remembering that good-quality phone cameras may capture it better than you can see it with the naked eye.

What is Comet S1 Atlas?

The second comet was spotted by astronomers in Hawaii on 27 September.

It was spotted by the ATLAS (Asteroid Terrestrial-impact Last Alert System), and belongs to the Kreuz family of comets, which pass extremely close to the sun and are known as ‘sungrazers’.

It will pass round the Sun and be visible around Halloween – if it survives.

Some reports suggest that it may already be disintegrating, however,

How can I see it?

It’s currently visible through telescopes in the constellation Hydra, and could be visible in the northern hemisphere after encountering the sun on 28 October.

If it survives its trip around the sun, the comet might be brighter than Venus in the night sky in late October, according to Star Walk.

If it survives its close encounter, it may also be visible to the naked eye, experts say – although some reports suggest the comet might already have ‘fallen apart’.

James Wray, Professor of Earth and Atmospheric Sciences, Georgia Institute of Technology, said: “The comet will pass closest to Earth on 24 October – look low in the eastern sky just before sunrise.

“Then, after swinging around the Sun, the comet may reappear in the western night sky right around Halloween. It’s possible, however, that it could disintegrate – in part or in whole – as sometimes happens when comets pass by the Sun – and this one will come within 1 million miles of our star.’